Neti neti is an ancient principle, predating Adi Shankara, indicating a meditative path of negating objects in consciousness.
The following is excerpted From Wikipedia, a long article that is well worth reading:
Neti Neti (Sanskrit : नेति नेति) is a Sanskrit expression which means "not this, not that", or "neither this, nor that". It is found in the Upanishads and the Avadhuta Gita and constitutes an analytical meditation helping a person to understand the nature of Brahman by negating everything that is not Brahman. One of the key elements of Jnana Yoga practice is often a "neti neti search." The purpose of the exercise is to negate all objects of consciousness, including thoughts and the mind, and to realize non-dual awareness. ...
Neti neti is indirectly referenced in Brahma Sutras III.2.22 and was cited by pre-Shankara commentators such as Sabara, Gauḍapada, and Mandana.[5] Adi Shankara was one of the foremost Advaita philosophers who advocated the neti-neti approach.
See: https://en.wikipedia.org/wiki/Neti_neti
The following is excerpted From Wikipedia, a long article that is well worth reading:
Neti Neti (Sanskrit : नेति नेति) is a Sanskrit expression which means "not this, not that", or "neither this, nor that". It is found in the Upanishads and the Avadhuta Gita and constitutes an analytical meditation helping a person to understand the nature of Brahman by negating everything that is not Brahman. One of the key elements of Jnana Yoga practice is often a "neti neti search." The purpose of the exercise is to negate all objects of consciousness, including thoughts and the mind, and to realize non-dual awareness. ...
Neti neti is indirectly referenced in Brahma Sutras III.2.22 and was cited by pre-Shankara commentators such as Sabara, Gauḍapada, and Mandana.[5] Adi Shankara was one of the foremost Advaita philosophers who advocated the neti-neti approach.
See: https://en.wikipedia.org/wiki/Neti_neti